1. Field of the Invention
The invention relates to a process for the selective hydrogenation of butadiene in crude high-butadiene C.sub.4 cuts to butenes. Such C.sub.4 hydrocarbon mixtures mainly occur when hydrocarbons originating from mineral oils, for example naphtha, are steam cracked. These hydrocarbon mixtures can contain, in addition to the chief component 1,3-butadiene, small amounts of compounds containing cumulenes and/or acetylenic triple bonds, which can also be selectively hydrogenated to butenes in the present process.
The composition of the crude C.sub.4 cut coming from the steam cracker can vary within wide limits (cf Table 1 below).
TABLE 1 ______________________________________ Two typical examples of the composition of C.sub.4 cuts leaving a steam cracker in percentages by weight I II ______________________________________ Butadiene 46 63 Butene-1 13 9 trans-Butene-2 4 3 cis-Butene-2 3 2 Isobutene 24 20 Isobutane 3 0.5 n-Butane 6 2 Vinylacetylene 1 0.5 ______________________________________
The composition is substantially dependent on that of the product to be cracked and on the cracking conditions in the steam cracker. The resulting crude C.sub.4 cut usually has a butadiene content of from 40 to 50% w/w.
Indeed, the process of the invention is capable of selectively hydrogenating any C.sub.4 cut having a butadiene content of up to 80% w/w, regardless of its origin.
2. Description of the Prior Art
Hitherto, such crude C.sub.4 cuts have been processed by methods involving the extraction of the butadiene with suitable solvents. Such methods are described in detail in, say, DE 2,724,365 and have been adopted for the operation of numerous industrial plants. The butadiene thus isolated can be used, for example, for making styrene-butadiene copolymers suitable, inter alia, for packaging foodstuffs.
In cases in which the separation of butadiene would be uneconomical, there is the alternative of selectively hydrogenating the butadiene to butenes. The latter form valuable products which can be processed to higher value-added chemicals.
Possible routes for such further processing of butenes are, for example:
conversion of isobutene to methyl-tert-butyl ether or tert-butylalcohol PA1 isolation of isobutene to serve as intermediate for polyisobutene or other products PA1 isolation of butene-1, for example by fractional distillation PA1 dimerization of n-butenes to octenes with subsequent conversion to plasticizer alcohols PA1 can be run using fixed-bed catalysts, PA1 causes the compounds to be hydrogenated to convert to simple compounds which remain so as far as possible, and PA1 can be operated without the addition of CO.
A basic prerequisite for the above upgrading processes, given by way of example, is the use of a virtually butadiene-free product having a maximum butadiene content of ca 0.2% w/w, as otherwise the formation of by-products in the form of undesirable oligomers and polymers will be too great.
Another use is, e.g., the copolymerization of ethylene and butene-1 to give so-called linear low-pressure and high-pressure polyethylenes, which are manufactured in large quantities on account of their improved application properties as compared with normal polyethylenes.
Such butadiene-free C.sub.4 cuts have hitherto been obtained only by the method of butadiene extraction, which reduces the butadiene content to about 1% w/w.
One conventional method of removing residual butadiene from butene-enriched C.sub.4 mixtures comprises selective catalytic hydrogenation. For example, DE 3,301,169 describes a process for the preparation of butadiene-free or virtually butadiene-free butene-1 from C.sub.4 streams having a butadiene content of up to 5% w/w by selective hydrogenation of the butadiene. The addition of small amounts of CO results in a selectivity of virtually 100%, even when hydrogenation is carried to very low residual butadiene contents of 10 ppm. This measure also greatly inhibits or prevents the isomerization of butene-1 to butene-2.
The selective hydrogenation of residual butadiene using added CO can be carried out by conventional methods in, e.g., the gas phase, liquid phase, or trickle phase. It is preferred to effect such selective hydrogenation of residual butadiene in the liquid phase or trickle phase using a fixed bed of hydrogenation catalyst.
EP 0,087,980 describes a method of carrying out the reaction in the gas phase, in which the hydrogen is fed to the reactor at at least two points.
DE 3,143,647 discloses a process for the selective hydrogenation of C.sub.3 and C.sub.4 hydrocarbons containing conjugated and cumulated double bonds and/or acetylenic triple bonds, by the use of fixed-bed supported catalysts, in which a) a finely distributed stream of hydrogen is homogeneously dissolved in the hydrocarbon to be hydrogenated before the latter enters the reactor, and b) at least 0.05 ppm w/w of CO must be added. This method makes it possible to hydrogenate butadiene-containing cuts having butadiene contents of up to 20% w/w.
Such hydrogenations can be carried out using conventional fixed-bed catalysts. Particularly suitable catalysts are metals in Subgroup VIII and Subgroup I of the periodic table, supported, for example, on aluminum oxide, pumice, clays, or silicates. TiO.sub.2 has also been proposed for use as a supporting material, cf EP 0,314,020.
Suitable catalysts, mainly for use in the selective hydrogenation of butadiene, pentadiene, and cyclopentadiene, are disclosed, e.g., in DE-A 3,207,029, DE-A 3,207,030, DE-A 3,143,647, and EP-A 0,011,906. These are supported palladium catalysts, the supports mainly being Al.sub.2 O.sub.3 or SiO.sub.2.
A significant improvement in the selectivity achieved in diene and acetylene hydrogenations can be obtained by partial poisoning, for example by adding promoters (Zn, Cd, Sn, Pb, or Hg) to the Pd-catalysts (cf G. C. Bond, Catalysis by Metals, Academic Press, London 1962, pp 99 and 297). More recently proposed promoters are, e.g., Ag (DE 3,119,850) and catalysts pretreated with inorganic bases (DE 2,849,026) or doped with alkali metal oxides or alkaline-earth metal oxides (R. J. Peterson, Hydrogenation Catalysts, Noyes Data Corp., New York 1977, p 183).
Besides the aforementioned addition of CO as a means of increasing the selectivity, it has also been proposed to modify catalysts with sulfur compounds. Thus it is possible to treat catalysts with thioethers to increase their selectivity toward acetylene, as proposed in FR 1,240,175. Finally, FR 2,355,792 discloses catalysts doped with hydrogen sulfide which are suitable for the selective hydrogenation of butadiene. However, they also catalyze the isomerization of, e.g., butene-1 to butene-2.
The aforementioned processes are unsatisfactory due to the facts that they are restricted to the use of products having low butadiene contents and/or that they require the addition of CO to increase the selectivity.
The use of compounds such as CO for increasing the selectivity is a disadvantage in the hydrogenation of high-butadiene streams since it necessitates the use of much higher reaction temperatures in order to achieve the desired conversions. However, high temperatures increase the occurrence of side-reactions (oligomerization, polymerization, overreaction to butane) and reduce the lifetime of the catalysts.